The present invention relates in general to circuit board apparatus used for implementing computing subsystems, and in particular to an apparatus including a baseboard and a family of reconfigurable daughtercards.
Printed circuit boards (PCBs) facilitate rapid construction of electronic circuits by providing a stable insulating sheet of material upon which circuit components are mounted. The components are typically interconnected using thin plated conductive (e.g. copper) lines bonded to the sheet that form circuit paths. A large variety of PCBs has been developed for many different applications.
Field programmable devices (FPDs) are circuits that include a large number of gates whose interconnection can be programmed to form a desired logic function. Utilizing any one of a variety of reprogrammable technologies, FPDs allow engineers to electrically program (and reprogram) standard, off-the-shelf logic elements to meet the specific needs of their application. Thus, FPDs combine the logistical advantages of standard, fixed integrated circuits with the architectural flexibility of custom devices. For the purposes of this description, the terminology "programmable device" refers to once programmable as well as reprogrammable devices.
Larger and more complex circuits can be developed by combining and interconnecting several FPDs on a PCB. A growing field of application for such boards is design and rapid implementation of prototypes for larger systems such as a microprocessor. Using computer aided design (CAD) tools and versatile FPD-based PCBs designers can develop, in a matter of hours, digital systems containing thousands of gates.
One example of an FPD-based PCB is the "Anyboard" developed at North Carolina State University. The Anyboard consists of a hardware card that incorporates six FPDs (more specifically field programmable gate arrays, or FPGAs), three random access memory (RAM) chips, buffers and local and global buses that connect the FPGAs to the RAMs and buffers. A complete description of the Anyboard system can be found in "Anyboard: An FPGA-based, reconfigurable system," IEEE Design & Test of Computers, pages 21-30, September 1992, by Van den Bout et al.
While FPD-based PCBs such as the Anyboard provide flexibility in terms of the variety of circuits that can be developed, to the extent that the architecture of the board is fixed in hardware the application of the board is limited. In case of the Anyboard, for example, the number of FPGAs and RAMs as well as the bus interconnection are fixed in hardware. This also limits the user's ability to upgrade and expand the system's hardware.
Altera Corporation of San Jose has developed a reconfigurable FPD-based universal circuit board (UCB) that provides significant improvements in terms of interconnect and architecture flexibility. The UCB includes a number of sockets that can receive FPDs, field programmable interconnect devices (FPIDs), or selective shorting circuits interchangeably. Various inter-socket interconnect topologies and board interface alternatives provide further flexibility. The UCB is fully described in a commonly assigned patent application, Ser. No. 08/206,774 filed Mar. 4, 1994 now U.S. Pat. No. 5,537,295, which is hereby incorporated by reference in its entirety.
While the UCB incorporates a sizable amount of memory on board, to add more memory or other types of logic blocks requires separate additional boards. This is also true if the application requires more than the maximum number of gates available on the UCB.
There is, therefore, a need for an improved circuit board apparatus that provides reconfigurability and increased flexibility in architecture, interconnectivity and ability to expand.